An Astronomer’s View of Optical Turbulence

1. An Astronomer’s View of Optical Turbulence The characterization, understanding and use of an astronomical site is a challenging affair René Racine Université de Montréal and Association of Canadian Universities for Research in Astronomy TMT site testing hardware

2. Summary • Futility • Utility • Challenges • Dreams

3. Futility

4. Declarations • The average integrated turbulence and turbulence profile above a site are highly predictable • Newton 1730 “Atop the Tallest Mountains, above the Grosser Clouds” • Racine 2005  [simple-minded model of FWHM(altitude, elevation)] • Their measurement for the purpose of siteselection is thus a futile activity • Factors besides optical turbulence can be decisive for site selection. • Politics : “landlords”, funding agencies, sponsors… • Enviro-cultural debates protracted delays • Costs This further enhances the futility of site testing and modeling for selection purpose.

5. Demonstrations

6. 44 pre-2005 and 7 2005 - 2007TMT night-timeseeing campaigns.Uniform, cross-calibrated TMT data less scatter Altitude dependence is weak The first few meters above ground are important Note: open circles are for Dome C

7. 2005-2007 TMT dataAverage deviation from model = 0.054” ( 0.037” w/o Tolar ) “The accuracy of the model is therefore estimated to be ±0.04".” Racine, R. 2005 PASP 117, 401 The accuracy of the TMT campaigns is therefore (0.0542 – 0.042)1/2 ~ 0.03”) (w/o Tolar ~ 0.00”)

8. Turbulence profiles differ less between sites than between nights or times. 7 sites, 83 night ~1000 profiles

9. Utility

10. What is important to astronomers is what the turbulence profile is or will be (now-casting, forecasting) • And what the resulting telescopic PSF will be • OT spectral index, outer scale effect • To adjust science programs to atmospheric conditions • Flexible, queue scheduling maximizes productivity • To adjust instrumental parameters to turbulence profile • Conjugate optics for AO, GLAO , MCAO…

11. Observatory managers care becausetelescope time is expensive One clearELT night $ 300 000 € 200 000

12. Astronomers care becausethe natural seeing is the most crucial determinant of the science SNR

13. Challenges

14. AO SNR 1 2 5 10 20 35 60 Challenge no. 1a : Forecast time dependent variationsfor the next night M. Sarazin 2006 Note again better seeing at higher elevation

15. The first few meters above ground areimportant.Is wind the main driver of ground OT ?

16. Challenge no. 1b : Forecast time dependent variationsover the loooong term AO SNR 0.2 0.5 5 30 6 months of DIMM seeing data at a TMT site

17. DIMM seeing (”) DIMM seeing (”) Challenge no. 2 : Explainscatter between sub-sites :very local effects are important Strictly simultaneous seeing measurements (8-m elevation, 2 min. samples) from two sub sites of same altitude separated by 600 m can differ very significantly. Seeing forecasts for that summit are largely irrelevant unless local effects are suitably modeled. A fitting interpretation is that each site is affected by uncorrelated sporadic local OT enhancements of probability of occurrence / mean amplitude : 50% / 0.15” Site A 25% / 0.65” Site B Analysis shows that differences in the DIMMs’ line-of sight (different stars) through the planetary boundary layer and the free atmosphere contribute a relative seeing dispersion of ~15%. 93 nights Site B Site A

18. One of many episodes of strong differences in seeing between two neighboring sites

19. Challenge no. 3: Explain “Dome and Mirror” OTEvolution of median image quality at the CFHT

20. CFHT image quality vs DTmirror DTdome Racine et al. 1991, PASP 103, 1020

21. All this local stuff may be “small stuff” to expert atmosphere dynamicists… • But it is of paramount importance to astronomers • Thus far, astronomers and their engineer colleagues have dealt largely empirically with these “mysterious” behaviors • Astronomers can take advantage of the research developed so far in OT. Close collaboration between astronomers and OT physicists, and coordinated benchmark tests should be very beneficial.

22. TMT enclosure and telescope structure: Efforts to minimize dome seeing mirror seeing wind buffeting

23. Dreams

24. What astronomers dream of • Clear skies ! • Excellent (<0.5”) seeing (integrated OT) • Foreknowledge of seeing • Mitigation of ground layer turbulence • Mitigation of “dome and mirror seeing” • Low altitude natural turbulence or • Foreknowledge of dominant layers’ altitudes • Low (<2 m /sec.) local winds (esp. for ELTs) • Low (<2 mm) PWV (for mid IR work)

25. Helping make these dreams come true • Clear skies : high altitude tropical (or polar?) site • reliable weather forecasts • Excellent seeing : high altitude site • reliable integrated OT forecast • Ground layer (< 30 m) turbulence • model ground-air interface, forecast how local OT arises • Dome and mirror seeing • ab initio quantitative models, mitigation measures • Foreknowledge of dominant layers’ altitudes • Reliable turbulence profile forecasts • Low (<2 mm) PWV • High altitude site • Low (<2 m /sec.) local winds (esp. for ELTs) • Low altitude site…or imaginative structures and wind shields

26. An obvious double-blind test • A DIMM in installed at a site, any site… • A team of OT experts • Inspects and instruments the site with ‘meteo’ sensors • Uses the data to calculate the zenith DIMM seeing • While an independent DIMM observer • Simultaneously measures the zenith seeing. • OT experts and DIMM observer do not communicate. • OT experts and DIMM observer transmit their time-flagged results to an independent reporter. • The calculations are improved until <|O – C|> < 0.05”. • Then the OT experts forecast the seeing for the next night…

27. To repeat… Astronomers are all aware that these dreams can only become reality through close collaboration with atmospheric physicists